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Problem solution · Python

Count the Number of Infection Sequences

Count the Number of Infection Sequences: a Python solution using dynamic programming. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Dynamic programming
Source
walkccc LeetCode Solutions
Length
34 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Dynamic programming

For Count the Number of Infection Sequences, the implementation records answers for smaller states and reuses them to build the requested result without repeating work.

  1. Define precisely what one DP state represents.
  2. Establish the base cases before transitions are evaluated.
  3. Process states in dependency order and combine only already-known values.

Code notes

  • 34 lines of Python from the credited upstream file 2954.py.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected.

Complexity

Multiply the number of reachable states by the work performed for each transition, then include the stored state table in memory usage.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeCount the Number of Infection Sequences · PythonPython
Use this to learn the idea, then write your own version.
class Solution:  def numberOfSequence(self, n: int, sick: list[int]) -> int:    MOD = 1_000_000_007     @functools.lru_cache(None)    def fact(i: int) -> int:      return 1 if i <= 1 else i * fact(i - 1) % MOD     @functools.lru_cache(None)    def inv(i: int) -> int:      return pow(i, MOD - 2, MOD)     ans = fact(n - len(sick))  # the number of infected children    prevSick = -1     for i, s in enumerate(sick):      # The segment [prevSick + 1, sick - 1] are the current non-infected      # children.      nonInfected = sick[i] - prevSick - 1      prevSick = sick[i]      if nonInfected == 0:        continue      ans *= inv(fact(nonInfected))      ans %= MOD      if i > 0:        # There're two choices per second since the children at the two        # endpoints can both be the infect candidates. So, there are        # 2^[nonInfected - 1] ways to infect all children in the current        # segment.        ans *= pow(2, nonInfected - 1, MOD)     nonInfected = n - sick[-1] - 1    return ans * inv(fact(nonInfected)) % MOD

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